Low-energy acoustic waves are created in solids when stresses in the solid cause sudden movement of fractures or zones of weakness. The energy release may be referred to as "acoustic emission" or, more commonly when the solid is a formation in the earth, the energy release is called a "microseismic event." Microseismic events may be caused by fluid pressure changes in the pore spaces of rock, which cause stress changes in the rock and movement at planes of weakness, or by the formation of hydraulic fractures. The seismic wave generated can be considered to be an extremely weak earthquake. It has been known for many years that such microseismic events occur in hydrocarbon reservoirs in which substantial pressure changes occur.
Hydraulic fracturing of wells is widely practiced as a method for increasing the production rate of the wells. In this method, fluid is injected at a high rate and at a pressure greater than the earth stress in the formation to be fractured. Typically, a vertical hydraulic fracture is created around a well, and the fracture may extend several hundred feet from the well. The fracture may also extend significant distances along the wellbore.
It is important to know the extent of a hydraulic fracture along a wellbore, so that it can be determined if the fracture has grown to intersect other permeable zones above or below the zone of interest. It is also desirable to know the length of the fracture away from the wellbore and the direction or azimuth angle of the fracture extending away from the well, so that the influences of the fracture on flow of fluids in the zone of interest can be predicted with greater accuracy.
It is also important to know whether a hydraulic fracture has penetrated an impermeable barrier layer during injection of a fluid into a well for disposal purposes. Such fluid may be a brine, radioactive material, or a hazardous chemical waste stream, for example. It is desirable to have a tool which can detect possible movement of the fluid and any solids it may contain out of the intended injection zone. Detection of microseismic events originating beyond an impermeable barrier which bounds the intended injection zone can indicate such movement.
Not surprisingly, a large number of microseismic events are associated with the hydraulic fracturing process. Several years ago it was found that an indication of hydraulic fracture direction or azimuth angle can be derived from microseismic events occurring soon after the hydraulic fracture is formed. Early work was reported by Dobecki in "Hydraulic Fracture Orientation Using Passive Borehole Seismics," Soc. of Pet. Engrs. Paper No. 12110, 1983. Data from microseismic events were analyzed to determine the polarization of the compressive wave (P-wave) from each event to determine azimuth direction of the event, the polarization being determined from a "hodogram." A hodogram is a plot of the output of a geophone in one direction versus the output of a geophone in another direction, such as the x-direction versus the y-direction. The distance from the event to the well was calculated by measuring the difference in arrival time of the P-wave and the shear wave (S-wave) at the well and multiplying this difference by a factor involving the respective P-wave and S-wave velocities. A histogram of the seismic events following hydraulic fracturing was also plotted in polar coordinates to indicate the azimuthal distribution of events and consequently, the direction of the hydraulic fracture.
Much more recently, examination of seismic events received in a well during hydraulic fracturing, pressure fall-off after fracturing, and flow-back of fluid was reported in "Acoustic Emission Monitoring During Hydraulic Fracturing," SPE Formation Evaluation Journal, pp. 139-144, June 1992. It was pointed out in this paper that when detecting microseismic events with a single set of triaxial geophones, an ambiguity of 180.degree. exists in the vertical or z-direction. The polarity of the first motion on arrival of a wave is not known because a source above or below the receiver may produce an identical signal.
A method of locating fractures from acoustic emissions received by single geophones placed in wells at a known distance from the well being fractured was reported in "Observations of Broad Band Microseisms During Reservoir Stimulation," Society of Exploration Geophysics 63rd Conference, Washington, 1993. This method is relatively expensive in that multiple wellbores must be used and multiple tools must be run. Triangulation calculations are used to locate the source of seismic events using the signals received in the separate wells.
Microseismic events may be produced in the subsurface by processes other than hydraulic fracturing of wells or pressure changes in a reservoir. Subsidence accompanying reservoir pressure reduction may also lead to movement of piles or other equipment at the surface or seabed above a reservoir, for example, producing additional microseismic events. Also, increase of pressure inside the casing of a well may cause mechanical failure of the cement sheath around the casing, and an acoustic wave may originate from very near the casing. If there is communication of fluid pressure along the wellbore outside the casing because of lack of a hydraulic seal by the cement, the pressure changes may cause microseismic events originating very near the casing.
Sources of acoustic waves in the subsurface are not limited to microseismic events. For example, a well flowing uncontrolled to the surface of the earth, called a "blowout," may flow at such high rates that significant acoustic noise is created at the bottom or at other segments of the well. There is often a need to locate the source of this noise in order to assist in attempts to stop the uncontrolled flow. Measurements of the source of the noise may be made from offset wells.
Wellbore acoustic receivers for detecting seismic waves have become widely available in recent years for Vertical Seismic Profiling (VSP) in wells. Typically, these wellbore acoustic receivers have three orthogonal seismic transducers (geophones or accelerometers) and include means for clamping the receivers against the casing of a well. In recent years, acoustic receivers suitable for seismic waves up to frequencies of 1000 Hz have been developed for cross-well seismic imaging. Such receivers, described in U.S. Pat. No. 5,212,354, may be used simultaneously at several levels, at intervals of about 10 feet between each receiver, to record seismic signals generated in another well. These seismic receivers use hydraulic pressure to clamp the receivers against casing with a high force compared with the weight of the receiver. A plurality of receivers may be used in a well, flexibly connected by hydraulic hose to other receivers and to the source of hydraulic pressure. The seismic signals are typically digitized and transmitted to the surface of the earth over conventional electrical wireline. Digitization of the downhole signals commences upon trigger activation of the "shot break" and continues for one or more seconds as data is stored in downhole memory. Subsequently, the data is pulsed to surface over a digital channel while the tool is inactive.
There is a need for improved apparatus and method to be used in a well to detect microseismic signals or other acoustic waves arriving at that well in real-time, with no periods of inactivity. The apparatus and method should decrease the ambiguity present in prior measurements; specifically, the 180.degree. ambiguity present when only one set of triaxial transducers is used in a well. To make possible real-time acquisition of data from multiple receiver units having triaxial transducers, improved apparatus and method for communicating additional channels of data to the surface are needed. Also, to assist in interpreting real-time microseismic activity around a well, means for communicating to the surface other downhole data, such as pressure, temperature, and hydrophone signals in the wellbore, should be available. Therefore, there is a need for means of telemetry of at least 6 and preferably 9 or more channels of data to the surface as acoustic waves around a well are generated and received. There is also a need for an improved method to process and allow interpretation of the data from the multiple receivers to provide greater accuracy in locating the sources of the acoustic waves. In addition, there is a need to determine whether a microseismic event originated above or below a specific location in a well. This information can be used, for example, to determine if a hydraulic fracture has formed from injection of fluid into a well and the fracture has penetrated an impermeable barrier confining the injection zone.